1. Field of the Invention
The present invention relates to a vehicle braking/driving force control apparatus for independently controlling a driving force and a braking force of each of front left and right and rear left and right wheels of a vehicle.
2. Description of the Related Art
Hitherto, there has been known a vehicle braking/driving force control apparatus for independently controlling a driving force and a braking force (both are collectively referred to as braking/driving force) for front left and right and rear left and right wheels of a vehicle. For example, there has been known a vehicle of an in-wheel motor type as a form of an electric vehicle, in which a motor is arranged inside or closely to the wheel of the vehicle, and the wheel is directly driven by the motor. In the vehicle of the in-wheel motor type, each motor is individually controlled for power running or regeneration, and a driving torque or a braking torque applied to each wheel is thus individually controlled, thereby generating a braking/driving force on each wheel.
Each wheel is suspended via a suspension to a vehicle body. In general, as illustrated in FIGS. 2A and 2B, an instantaneous rotation center Cf of a suspension for coupling a front wheel 10f to a vehicle body B is positioned behind and above the front wheel 10f, and an instantaneous rotation center Cr of a suspension for coupling a rear wheel 10r to the vehicle body B is positioned ahead of and above the rear wheel 10r. Therefore, as illustrated in FIG. 2A, when a driving torque is applied to the front wheel 10f, a force Ff1 forward in a travel direction of the vehicle acts at a ground contact point of the front wheel 10f, and a vertical force Fzf1 biasing the vehicle body B downward is generated by the force Ff1 at the ground contact point of the front wheel 10f. Thus, the force in the direction of sinking the vehicle body B acts as a result of the driving of the front wheel 10f. In contrast, as illustrated in FIG. 2B, when a braking torque is applied to the front wheel 10f, a force Ff2 backward in the travel direction of the vehicle acts at the ground contact point of the front wheel 10f, and a vertical force Fzf2 biasing the vehicle body B upward is generated by the force Ff2 at the ground contact point of the front wheel 10f. Thus, the force in the direction of raising the vehicle body B acts as a result of the braking of the front wheel 10f. 
On the other hand, the generation direction of the vertical force on the rear wheel 10r is opposite to that for the front wheel 10f. In other words, as illustrated in FIG. 2A, when a driving torque is applied to the rear wheel 10r, a force Fr1 forward in the travel direction of the vehicle acts at a ground contact point of the rear wheel 10r, and a vertical force Fzr1 biasing the vehicle body B upward via the suspension is generated by the force Fr1 at the ground contact point of the rear wheel 10r. Thus, the force in the direction of raising the vehicle body B acts as a result of the driving of the rear wheel 10r. In contrast, as illustrated in FIG. 2B, when a braking torque is applied to the rear wheel 10f, a force Fr2 backward in the travel direction of the vehicle acts at the ground contact point of the rear wheel 10r, and a vertical force Fzr2 biasing the vehicle body B downward via the suspension is generated by the force Fr2 at the ground contact point of the rear wheel 10r. Thus, the force in the direction of sinking the vehicle body B acts as a result of the braking of the rear wheel 10r. 
In this way, a part of the longitudinal braking/driving force on the wheel is converted by the suspension into the force in the vertical direction of the body B. Thus, motion control (including a body attitude control such as restraint of a roll motion, a pitch motion, and a heave motion) for the vehicle can be carried out by controlling a braking/driving force on the each wheel. Therefore, a target braking/driving force on the each wheel is calculated as a resultant force of a driver-requested distributed driving force acquired by distributing a driver-requested driving force set in response to an operation amount by a driver to the each wheel and a control braking/driving force for carrying out the motion control for the vehicle.
Based on this view point, for example, a vehicle travel control apparatus proposed in Japanese Patent Application Laid-open No. 2009-143310 controls a braking/driving force of each in-wheel motor to restrain a roll motion of the vehicle.
By the way, when the motion control for the vehicle is carried out by the braking/driving forces on the wheels, the direction of the target braking/driving force may be inverted depending on the motion state of the vehicle. For example, while the control braking/driving force is opposite in direction to the driver-requested distributed driving force, when a relationship in magnitude between both forces changes, the direction of the target braking/driving force is inverted. In this case, the motor is switched from a state (powering) where the driving force is generated on the wheel to a state (regeneration) where the braking force is generated on the wheel, or is switched from the state (regeneration) where the braking force is generated on the wheel to the state (powering) where the driving force is generated on the wheel. Thus, an inversion of the motor torque (so-called zero cross of the motor torque) occurs. Therefore, when the motor torque is transmitted via speed reduction gears to the wheel, a noise is generated when a backlash is taken up by the inversion of the motor torque.